In optical fiber manufacturing, during the drawing process, a coating resin is coated around the circumference of a silica glass fiber immediately after the fiber is drawn to prevent strength reduction of the resulting optical fiber. An ultraviolet-curable resin, particularly a urethane-acrylate-type or an epoxy-acrylate-type, is generally used as the coating resin for the optical fiber.
An optical fiber increases transmission loss due to external stress and microbends caused by such external stress. To protect the optical fiber from external stress, the optical fiber is usually coated with two layers (a soft layer and a hard layer) of coatings. For the inner layer, which directly contacts the silica glass, a soft resin with low Young's modulus is used as a buffer layer (hereafter a primary layer), and for the outer layer, a hard resin with high Young's modulus is used as a protective layer (hereafter a secondary layer). Conventionally, a resin with 3 MPa or less in Young's modulus is used as the primary layer, and a resin with 500 MPa or more in Young's modulus is used as the secondary layer.
In a typical optical fiber manufacturing method, a silica glass fiber is drawn from a silica glass preform by heating the bottom portion of the preform in a drawing furnace; then, a primary layer and a secondary layer are coated by applying liquid ultraviolet-curable resins via coating dies to the drawn silica fiber, and by curing the resins with ultraviolet light.
In the next process, a colored optical fiber is created by applying a color layer made from a colored resin around the circumference of the optical fiber. The structure of the optical fiber is shown in FIG. 1. In this specification, a silica-glass fiber coated with a primary layer and a secondary layer is called an optical fiber. An optical fiber coated with a color layer made from a colored resin around the circumference of the optical fiber is called a colored optical fiber, and multiple optical fibers positioned in a planar array and bound together with a ribbon resin is called an optical fiber ribbon. Also, the term “coated optical fiber” includes glass fibers with coatings; more specifically, it includes an optical fiber; and a colored optical fiber.
When such a coated optical fiber is immersed in water, its transmission loss may increase. To create highly reliable coated optical fibers, which do not increase their transmission loss even if the coated optical fiber is immersed in water for a long period of time, various suggestions such as improvement in adhesive strength between a primary layer and a glass fiber, have been made.
As the popularity of optical fibers is growing in recent years, the number of optical fiber cable applications is increasing, which indicates that the environments where optical fiber cables are used has been diversified, and new cable structures are being developed. Because of that, the long-term reliability required for optical fiber cables becomes stricter. With the situation as stated above, a coated optical fiber, which is less likely to increase its transmission loss when it is immersed in water, is being considered. However, it has a limit in handling the above-stated issue by controlling the adhesive strength between each layer; and currently such technology is used in combination with structures, which avoid water to reach the coated optical fiber or reduce the amount of water reaching the coated optical fiber by changing cable structures, cords and/or sheath material. However, these approaches do not offer sufficient reliability.